The present invention relates to a method and apparatus for the non-invasive measurement of internal particulate temperature utilizing ultrasonic tomography techniques.
The food processing industry currently utilizes two different processes for the sterilization and preparation of food products. The first method is commonly referred to as the conventional canning method and has seen little change in nearly 100 years. In the conventional canning method, a container is filled with food materials and sealed. The next step is the retort which is commonly performed in batches. The retort is accomplished by heating the food material to temperatures in the range of 120.degree. to 150.degree. C. (250.degree. to 300.degree. F.) and maintaining the temperature for specified periods of time, typically 5 to 15 minutes. Although the retort step cooks the food, its primary purpose is the destruction of harmful organisms, most notably those responsible for botulism. The time/temperature requirements for a particular food material is determined experimentally for each plant and process, but is also subject to a number of practical constraints which introduce the need for safety factors.
After retort the food product is cooled and a representative number of containers are punctured to determine the internal temperature by the use of a thermometer or thermocouple. The temperature measurement does not assess the internal particulate temperature, but only serves to verify that the process parameters are within acceptable limits. In order to assure that the particulate temperature has been elevated and maintained for an adequate period of time, extremely conservative safety factors are added to the process, such as, processing at a higher temperature or for a longer period of time, or both. As a result, product quality is degraded by overcooking, and a substantial amount of energy is wasted in the process. Additionally, containers that have been punctured must be disposed of at a significant cost to the food processing industry, costs which are ultimate borne by the consumers.
Another method of processing food is called the aseptic process, in which the food materials are pumped continuously through a heat exchanger in order to raise the temperature of the material to a predetermined level and maintain the temperature for a prescribed period of time. The food product is then cooled and finally placed and sealed in pre-sterilized containers. The aseptic process typically uses 10 to 20% of the energy of the equivalent conventional process and produces a better quality product because it does not overcook the product as the conventional canning method does. However, in the United States the aseptic process is only used for products consisting of a single phase, for example, fruit juices and other liquids.
In single phase materials, temperatures can be measured simply and accurately at any desired location in the heat exchanger by thermocouples or similar devices. However, the residence time in the heat exchanger is not well defined since the product which travels through the heat exchanger moves at different speeds, depending upon its proximity to the walls of the heat exchanger. The aseptic process has not yet been approved by the Food and Drug Administration for food materials containing particulates, i.e., multi-phase materials, due to the uncertainties of the temperature of the particulates in the food material and the residence time.
Ultrasonic temperature devices are known which have the capability for the non-invasive measurement of temperature in a single medium. These devices are based upon the principle that the speed of sound in any medium is a function of temperature. In a single phase medium, a through-transmission measurement is performed using one acoustic transducer as a sending element to generate short pulses of sound, and a second transducer to receive the sound pulses. The transmission time between the two is a measure of the speed of sound.
However, when measurements are made on two (or multi-) phase media, the problems involved in relating the measurements to the physical system are far greater. (see C. Javanaud, "Applications of Ultrasound to Food Systems", Ultrasonics, vol. 26, pp. 117-123, May 1988). The problem arises because the speed of sound at a given temperature is a unique function of each material. In general, the speed of sound is different in one material as opposed to another, even when the two materials are at the same temperature. When the acoustic flight path of an ultrasonic pulse is partly in one medium and partly in another, for example liquid and solid, the time of flight is proportional to the average speed of sound in each medium along the flight path. If the location and/or the dimensions of the solid or semi-solid particles along the transmission path are not known, a unique value for the speed of sound through the particle cannot be determined.
Therefore, it is necessary to know the length of flight in each medium, so that the speed of sound and thus the temperature in each medium is known from the overall measurement of the transit time of the ultrasonic pulse. Tomography techniques can be used to determine the length of flight in each medium, and therefore the temperature at a specific point in a non-homogeneous material. The use of ultrasonic tomography for in-process measurements of particulate temperatures in the food processing industry will result in lower energy requirements, reduced costs associated with disposal of unusable product, and will result in a better quality product since overcooking is avoided. Additionally, ultrasonic tomography can be used by the food processing industry for determining the in-process particulate temperatures in freezing operations, thereby reducing energy requirements and costs.
It is an object of this invention to provide a device that measures the internal temperature of particulates during processing.
It is another object of this invention to provide a device that measures the temperature of a multi-phase material at any location within the material.
It is another object of this invention to provide a method for non-invasively determining the temperature in a multi-phase material.
Additional objects, advantages and novel features of the inventions will become apparent to those skilled in the art upon examination of the following and by practice of the invention.